Contents

Abstract

We experimentally identiﬁed the fatigue crack growth micromechanisms operating near the limit plasticity regime in a commercial high-pressure die-cast AM50 alloy with a critical review of the available literature. An existing multistage fatigue model was modiﬁed to subsequently recognize these micromechanisms in a threefold fatigue crack growth regime. The formation of the main fatigue crack occurred almost exclusively at shrinkage pores and to a lesser extent at large Mn-rich particles. At shrinkage pores, several adjacent cracks typically incubated at the edge of ﬂat interdendritic pores, propagated along the a-Mg dendrite cells/Al-rich eutectic interface, and rapidly coalesced into a main physically small fatigue crack that advanced through the Al-rich eutectic. In the long crack regime, the crack advanced in a mixed transdendritic–interdendritic mode along persistent slip bands spreading over several tens of dendrite cells. The model predicts well the fatigue life compared to the experimental data when these observed mechanisms are accounted for.

Introduction

The high strength-to-weight ratio of contemporary magnesium alloys that are suitable for ultimate weight reduction purposes in automotive and aircraft components has stimulated in the last decade substantial interest in understanding their fatigue crack growth behavior with process rationalization. Experimental observations on cast Mg alloys have accumulatively revealed that the dendrite cell size, pores, secondary phase particles, persistent slip bands and twinning in the dendrite cells considerably aﬀect the fatigue durability and crack growth mechanisms of dendritic magnesium alloys [3–7].

Our interest of studying the AM50 alloy was motivated by the universal trend to reduce the aluminum content of Mg–Al alloys in an eﬀort to increase ductility under impact and fatigue loading for automotive components. In this paper, relevant micromechanisms of fatigue crack growth in an AM50 alloy are carefully examined. Fatigue tests were carried out in a fully reversed strain control condition at strain amplitudes ranging from 0.1% to 0.7%. The microstructure before and after fatigue failure was analyzed by means of ﬁeld emission gun-scanning electron microscopy (FEG-SEM) and electron probe microanalysis (EPMA) using specimens extracted from the grip regions and gage lengths of tested samples, respectively.